Microalgae are marine or freshwater unicellular organisms capable of performing photosynthesis, i.e. using daylight as an energetic source to fix carbon dioxide to grow and thus release oxygen. Thus, microalgae are photoautotrophic organisms.
Microalgae produce approximately half of the atmospheric oxygen. Microalgae are also important in the food chain. They are the main foodstuff for fish, mollusk and other aquatic animals.
Microalgae are at the heart of many research and programs and many start-ups have been launched with the aim of developing and exploiting their capacities.
Microalgal production worldwide has doubled in five years, from 5000 tons to 10000 tons (van Harmelen & Oonk, 2006).
There are thousands of catalogued microalga species, although only a few of them are commercially exploited. The main requirements a microalga species must meet to be susceptible of industrial use are suitable growth and a different biochemical composition conferring it the highest possible added value. In this sense, the microalga species that are commercially exploited today range from Chlorella and Nannochloropsis for aquaculture (Borowitzka, Journal of Biotechnology, 70(1-3), (1999) 313-321) to Spirulina for human consumption (Morist et al., Process Biochemistry, 37(5), (2001), 535-547), or Dunaliella and Haematococcus for the production of carotenoids such as beta-carotene and astaxanthin, respectively (Guerin et al., Trends in Biotechnology, 21(5), (2003) 210-216).
Microalgae require very few elements to grow, water, nutrients and Sun.
Under particular conditions, microalgae species are known to accumulate fatty acids up to 80% of their dry weight (Christi 2007).
When microalgae growth happens under stress conditions, such as nitrogen deficiency, silica deficiency or other nutrient deficiencies, microalgae accumulate high quantities of neutral lipids, especially triacylglycerol (TAG).
TAG are called neutral lipids because they don't carry charged group, contrary to phospholipids or glycolipids. TAG are apolar lipids. Thus, they are insoluble in water.
TAG are a combination of 3 fatty acids and a glycerol molecule. They are stored in vegetal organisms and animal fat tissues and mainly used as an energy reserve.
TAG can have different industrial uses. A growing use of TAG is the production of biodiesel by a transesterification but for the moment, this use is very expensive.
Fatty acids can be saturated, monounsaturated or polyunsaturated.
Poly unsaturated fatty acids (PUFAs) are the most interesting fatty acids, because of their benefits for human health. They have properties that permit to lower cholesterol level and atherogenesis risk.
Among the PUFAs of interest potentially produced by microalgae, there are omega-3 fatty acids. Omega-3 fatty acids are Cis-polyunsaturated fatty acids. They are essential fats, that means that the human body can't synthetize them.
In particular, docosahexaenoic acid (DHA) has been identified as having a major role in the prevention of many of the ailments which afflict modern society eg ischaemic heart disease, rheumatoid arthritis and associated conditions and the degeneration of tissue function. The specific role of DHA has been identified and it is accepted that dietary supplementation with DHA is desirable.
Another omega-3 of interest is eicosapentaenoic acid (EPA). EPA has a major role in the production of prostaglandins, molecules that control blood clotting and other arterial functions. Like DHA, EPA is also known for its benefits for the cardiovascular system.
However, the nutrients deficiencies or the other stress that undergo microalgae to enhance their lipid production often stop their growth and the lipid production at the same time and lead to the consumption of the lipids produced during the growth.
Thus, there remains a need in the art for a way to enhance lipid production in microalgae, preferably production of neutral lipids.
The inventors have obtained a microalgal strain naturally rich in lipids, in particular neutral lipids, which fits to this need.